Method and apparatus for improved signal to noise ratio in raman signal detection for mems based spectrometers

ABSTRACT

A method of Raman detection for a portable, integrated spectrometer instrument includes directing Raman scattered photons by a sample to an avalanche photodiode (APD), the APD configured to generate an output signal responsive to the intensity of the Raman scattered photons incident thereon. The output signal of the APD is amplified and passed through a discriminator so as to reject at least one or more of amplifier noise and dark noise. A number of discrete output pulses within a set operational range of the discriminator is counted so as to determine a number of photons detected by the APD.

BACKGROUND OF THE INVENTION

The invention relates generally to spectroscopy systems and, moreparticularly, to a method and apparatus for improved signal-to-noiseratio in Raman signal detection for micro electromechanical system(MEMS) based spectrometer devices.

Spectroscopy generally refers to the process of measuring energy orintensity as a function of wavelength in a beam of light or radiation.More specifically, spectroscopy uses the absorption, emission, orscattering of electromagnetic radiation by atoms, molecules or ions toqualitatively and quantitatively study physical properties and processesof matter. Raman spectroscopy relies on the inelastic scattering ofintense, monochromatic light, typically from a laser source operating inthe visible, near infrared, or ultraviolet range. Photons of themonochromatic source excite molecules in the sample upon inelasticinteraction, resulting in the energy of the laser photons being shiftedup or down. The shift in energy yields information about the molecularvibration modes in the system/sample.

However, Raman scattering is a comparatively weak effect in comparisonto Rayleigh (elastic) scattering in which energy is not exchanged.Depending on the particular molecular composition of a sample, onlyabout one scattered photon in 10⁶ to about 10⁸ tends to be Ramanshifted. Because Raman scattering is such a comparatively weakphenomenon, an instrument used to analyze the Raman signal should beable to substantially reject Rayleigh scattering, have a high signal tonoise ratio, and have high immunity to ambient light. Otherwise, a Ramanshift may not be measurable.

A challenge in implementing Raman spectroscopy is separating the weakinelastically scattered light from the intense Rayleigh-scattered laserlight. Conventional Raman spectrometers typically use reflective orabsorptive filters, as well as holographic diffraction gratings andmultiple dispersion stages, to achieve a high degree of laser rejection.A photon-counting photomultiplier tube (PMT) or a charge coupled device(CCD) camera is typically used to detect the Raman scattered light.

Concurrently, however, there is a growing need for miniaturization ofinstruments for biological, chemical and gas sensing in applicationsthat vary from medical to pharmaceutical to industrial to security. Thisis creating a paradigm shift in experimentation and measurement, wherethe trend is to bring the instrument/lab to the sample rather thanbringing the sample back to the lab for analysis. Unfortunately,conventional methods of Raman detection that also provide sufficientsignal-to-noise ratio (e.g., PMTs, CCD cameras) are generally notcompatible with MEMS scaled devices, due to the bulk associatedtherewith. Accordingly, it is desirable to be able to detect Ramanscattering with a higher signal-to-noise ratio given the constraints ofsmaller, on-chip “in the field” spectrometer devices.

BRIEF DESCRIPTION OF THE INVENTION

The above and other drawbacks and deficiencies of the prior art may beovercome or alleviated by an embodiment of a method of Raman detectionfor a portable, integrated spectrometer instrument, including directingRaman scattered photons by a sample to an avalanche photodiode (APD),the APD configured to generate an output signal responsive to theintensity of the Raman scattered photons incident thereon. The outputsignal of the APD is amplified and passed through a discriminator so asto reject at least one or more of amplifier noise and dark noise. Anumber of discrete output pulses within a set operational range of thediscriminator is counted so as to determine a number of photons detectedby the APD.

In another embodiment, an apparatus for Raman detection within aportable, integrated spectrometer instrument includes an avalanchephotodiode (APD) integrated on a chip, the APD configured to receiveRaman scattered photons by a sample and generate an output signalresponsive to the intensity of the Raman scattered photons incidentthereon. An amplifier is integrated on the chip, the amplifierconfigured to amplify the output signal of the APD and pass theamplified output signal through a discriminator integrated on the chipso as to reject at least one or more of amplifier noise and dark noise.A digitizer is integrated on the chip, and configured to count a numberof discrete output pulses within a set operational range of thediscriminator so as to determine a number of photons detected by theAPD.

In another embodiment, a method of Raman detection for a portable,integrated spectrometer instrument includes directing an input opticalbeam incident upon a sample to be measured, the input optical beammodulated at a heterodyne frequency. Photons scattered by the sample aredirected through receiving optics so as to filter Rayleigh scatteredphotons and pass Raman scattered photons through a tunable filter, andthe passed Raman scattered photons are detected at a wavelength passedby the tunable filter through demodulation at the heterodyne frequency.

In still another embodiment, an apparatus for Raman detection within aportable, integrated spectrometer instrument includes an optical sourcefor directing an input optical beam incident upon a sample to bemeasured. A beam-interrupting mechanism is configured to modulate theinput optical beam at a heterodyne frequency, and receiving optics areconfigured to collect photons scattered by the sample so as to filterRayleigh scattered photons and pass Raman scattered photons through atunable filter. A photon detector is configured to detect the passedRaman scattered photons at a wavelength passed by the tunable filterthrough demodulation at a heterodyne frequency.

These and other advantages and features will be more readily understoodfrom the following detailed description of preferred embodiments of theinvention that is provided in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an exemplary portable, hand-held Ramanspectrometer device, suitable for use in accordance with an embodimentof the invention.

FIG. 2 is a schematic block diagram of a MEMS-based Raman signaldetection system associated with the portable, hand-held Ramanspectrometer device of FIG. 1.

FIG. 3 is a block diagram illustrating a method for Raman detectionthough photon counting for improved signal-to-noise ratio, in accordancewith an embodiment of the invention.

FIG. 4 is a schematic diagram illustrating one possible implementationof an acousto-optic modulator used for heterodyne Raman signaldetection, in accordance with a further embodiment of the invention.

FIG. 5 is a schematic diagram of a MEMS cantilever configured forchopping an input optical beam for heterodyne Raman signal detection, inaccordance with a further embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention disclosed herein include a method forimplementing high signal-to-noise ratio (SNR) in Raman signal detectionfor micro electromechanical (MEMS) based spectrometer devices,characterized generally in one aspect by photon detection through acooled avalanche photodiode used in conjunction with photon counting.Other embodiments include chopping of the incident optical beam forheterodyne detection by modulating the source with a reference frequencyand then extracting the signal by demodulating the output at thereference frequency. Chopping may be implemented, in one embodiment,through an acousto-optic modulator (AOM) crystal for diffracting andshifting the incident beam away from the sample at the heterodynemodulation frequency, or through an MEMS cantilever that deflectsto/from the path of the input beam at the modulation frequency. Stillother embodiments include a combination of heterodyne detection andphoton counting for even further SNR improvement.

Referring initially to FIG. 1, there is shown a perspective view of acompact, hand-held spectrometer device 100 suitable for use inaccordance with an embodiment of the invention. It should be understoodthat the hand-held spectrometer device 100 illustrates only an exemplaryapplication/environment for the inventive embodiments discussed hereinand is intended to demonstrate disclosed improvements in detecting Ramanscattering (through higher signal-to-noise ratio), given the constraintsof smaller, on-chip “in the field” spectrometer devices such as theexemplary device shown in FIG. 1.

For the exemplary device 100 depicted, a hand-held body 102 includeslarge, user-friendly operator controls 104, a display portion 106 withan optional audio alarm feature 108, and an optional, disposableoptofluidics module 110 having a fluid inlet 112 for collecting andanalyzing a fluid sample. Although not specifically illustrated in FIG.1, the device 100 also has an optical signal output port through whichan incident beam is directed, and an optical signal input port 116through which a reflected beam is received. Additional informationregarding such hand-held spectrometer devices may be found in U.S.patent application Ser. No. 11/400,948, having the same assignee as thepresent application.

FIG. 2 is a schematic block diagram of at least a portion of the opticalcomponents used in the exemplary MEMS based, Raman detection deviceshown in FIG. 1. An optical source 202 (e.g., laser) directs incidentphotons at sample 204, such as one contained in module 110 of FIG. 1,for example. In an exemplary embodiment, a scattered signal from thesample 204 is collected (e.g., at an angle of about 90 degrees withrespect to the incident beam) through a micro lens 206 which is mountedon piezo-based mount (not shown), for example, so as to adjust the focalspot and to maximize the Raman signal and minimize the Rayleighscattered light and transmitted laser light. The Raman scattered lightcollected by the high NA (numerical aperture) lens is coupled with theoptical fiber in a Fiber Bragg grating (FBG) 208 having a transmissionwavelength that may be fixed by the pitch of the FBG 208.

In addition, a tunable Fabry-Perot cavity 210 is provided for filteringthe received Raman scattered photons at a selected wavelength, with thefiltered Raman photons directed to a sample (S) detector 212. Also shownin FIG. 2 is a beam-interrupting mechanism 214 that chops (modulates)the intensity of the incident beam upon the sample 204, for purposesdescribed in further detail hereinafter. In the case of heterodynedetection, the modulation frequency of the beam-interrupting mechanismis used to demodulate the signal detected by detector 212. A portion ofthe (modulated) input beam may be directed toward a reference (R)detector 216.

Referring now to FIG. 3, there is shown a schematic block diagram of aphoton counting apparatus 300 and method for detecting a Raman scatteredsignal from a MEMS based spectrometer device, in accordance with anexemplary embodiment of the invention. As is shown, individualcomponents of the photon counting apparatus 300 (which represents anexemplary embodiment of sample detector 212 in FIG. 2) are integrated onan individual microchip 302, which includes an avalanche photodiode(APD) 304, preamplifier 306, amplifier 308, discriminator 310, digitizer312 and processor 314.

Raman scattered photons (arrows 316) from a sample (e.g., sample 204 inFIG. 2) are incident upon the APD 304, which is cooled (e.g., bythermoelectric cooling) to a temperature of about −60° C. and which isbiased at a voltage slightly less than the APD's breakdown voltage (alsoknown as the “sub-Geiger” mode of operation). Suitable examples of suchan APD include, but are not limited to, model C 30645E from Perkin Elmerand model NDL 5553P from NEC Corporation. An output current pulse fromthe APD 304 is shaped by a preamplifier 306 (e.g., such as ORTEC model9306, 1 GHz preamplifier) so as to create a NIM (Nuclear InstrumentationMethods) standard pulse. In turn, the NIM pulse is received by thehigh-gain amplifier 308 where it is converted from a current signal to acorresponding voltage signal.

The amplified voltage signal output from the high-gain amplifier 308 iscoupled to the discriminator 310 so as to isolate single photon signalscorresponding to the voltage pulses within the range setting (e.g.,about 50 mV to about 1 V), with the analog output signal therefrom thenconverted to a digital signal by digitizer (A/D) converter 312. Theindividual photon counts are tracked/counted by the processor 314 andscaled as counts per second (cps) versus wavelength.

The minimum detectable power using the APD 304 depends upon the quantumefficiency of the APD 304 at a particular wavelength, the integrationtime, the dark current generation probability, the probability ofdetections against the discriminator setting, and the ionizationcoefficient (k_(eff)) of the APD. In particular, the higher theintegration time and the lower the ionization coefficient, the lower theminimum detectable power. By way of example, with an integration time ofabout 10 milliseconds, an ionization coefficient of about 0.005 at a1000 nm operating wavelength, the minimum detectable power is on theorder of about 3 femto-watts, at least an order of magnitude less thanconventional detection capabilities of portable devices.

The use of the APD in the sub-Geiger mode of operation provides certainadvantages over the Geiger mode of operation (i.e., the APD biased abovethe breakdown voltage), in terms of the exemplary embodiments presentedherein. First, the degree of APD gain is controlled in Sub-Geiger mode.In addition, there is no after-pulse effect that will limit the fasteroperation of the APD, unlike the Geiger mode. Furthermore, the heatingof the APD, due to the avalanche process of ions, gives rise to a higherthermal noise in Geiger mode and, as such, reduces the SNR.

As a Raman scattered photon represents a very low intensity signal, thenoise factor arising from APD dark current, amplifier noise andbackground noise are to be taken into account in order to attain a highSNR. Accordingly, the APD 304 is cooled to about −60° C. while the laser202 is further modulated by the beam-interrupting mechanism 214. Thesample detector 212 is further gated so as to be synchronized with thebeam-interrupting mechanism 214 such that it only detects when the laseroutput is actually incident upon the sample 214. Because the amplifiedNIM pulse is passed through the discriminator 310 (FIG. 3), theamplifier noise, APD noise and dark noise can all be rejected, therebyimproving SNR.

Moreover, for a highly fluorescent sample molecule (where Raman cannotbe seen due to a very high intensity of fluorescence spectra), thepresent detection scheme is particularly useful as a fluorescence-eventtime scale is on the order of about a few microseconds to about a fewmilliseconds. Conversely, Raman scattering is instantaneous wherein thedetection in this case is accomplished when the optical beam is actuallyincident on the sample. Thus, the detection process is alsoinstantaneous such that only the Raman signal can be detected. Becausethe detector 212 may be gated, time resolved studies for short livedreactions or transient reactions may be carried out by changing thegated time of the detector.

Referring once again to FIG. 2, during portions of a duty cycle (e.g.,50%) the beam generated by the laser 202 is passed through the mechanism214 and is incident upon the sample 204. As indicated above, both theRaman scattered signal and background light is collected through thecollecting lens 206 and transmitted through the optical fiber and FBG208 so that Rayleigh scattered light is filtered. Thus, only the Ramanscattered light is transmitted through the FBG 208 and to the input ofthe tunable Fabry-Perot cavity 210.

On the other side of the Fabry-Perot cavity 210, the TEC-cooled APD 304(FIG. 3) is coupled thereto through a high numerical aperture (NA) lens(not shown) with minimal loss. When the beam-interrupting mechanism 214is modulating with 50% duty cycle, the laser output will be incidentupon the sample 204 for the first portion of the cycle (e.g., 10 ms),and a first signal “A” will be detected by the APD 304. This firstsignal “A” contains both a Raman signal and a background signal.Conversely, when the laser is “chopped” (i.e., “off” during the secondportion of the duty cycle), then only a background signal “B” isdetected. The true Raman signal is therefore the difference between thefirst and second signals (i.e., A-B).

For embodiments where a beam-interrupting mechanism 214 is notimplemented in conjunction with the photon counting method of Ramandetection, background noise and noise from the amplifier 308 will be theprimary obstacles in attaining higher SNR. Thus, the amplified signalfrom the APD is fed to the discriminator 310 where the lower levelcutoff voltage thereof is set such that the short noise from theamplifier and the dark current from the APD will be cut off. Further,cosmic rays and very intense Rayleigh scattered signals are rejectedthrough proper setting of the upper level cutoff voltage of thediscriminator 310.

In accordance with another exemplary embodiment of the invention, aheterodyne technique for Raman signal detection is also disclosed for ahand-held Raman micro spectrometer, such as generally depicted by thebeam-interrupting mechanism 214 shown in FIG. 2. In this regard,heterodyne signal detection includes modulating the input signal source(laser) 202 at a reference frequency (e.g., 15 kHz) and then extractingthe reflected signal by demodulating the output at the referencefrequency.

The present embodiments depict at least two approaches by which suchmodulation may be implemented: (1) through the use of an acousto-opticmodulator (AOM) crystal, and (2) by mechanically chopping the sourcelaser beam using a MEMS cantilever. The Raman signal received at thedetector placed at the output of the tunable cavity would be demodulatedat the reference frequency using a simple lock-in detection circuit.Accordingly, this results in significant signal-to-noise improvementgiven that Raman signals are typically very weak (e.g., a few picowatts) and also mitigates issues such as power management at the samplechamber/sample itself.

Because MEMS based IR/Raman spectrometers are not very common, the mostprevalent technique to mitigate a power management problem is throughthe use of pulsed sources that are considerably expensive. Accordingly,as once again depicted in FIG. 2, light from the laser source 202 maychopped by the beam-interrupting mechanism 214, embodied for exampleusing an acousto-optic modulator crystal or a scanning MEMS basedcantilever at a frequency of a few kHz.

As more particularly illustrated in FIG. 4, an acousto-optic modulator(AOM) 400 includes an acousto-optic crystal 402 that acts as adiffraction grating when an acoustic wave (RF) is launched therein. Theinput laser beam 404 that passes though the crystal 402 is thendiffracted, resulting in a changed wavelength, as well as spatialposition for the higher-order diffracted beams with respect to thezero-order (spatially unshifted) beam. For a certain angle of incidenceon the crystal and RF power, most of the laser power can be transferredfrom the zero order beam 406 (unshifted in frequency and position) tothe first order-diffracted beam 408.

The AOM 400 can thus be used to switch on and off the laser beam 404incident on the sample chamber 412 by triggering the RF drive frequencyusing a TTL pulse (corresponding to the modulation frequency) that willcause the laser power to be shifted from the zero order to first orderbeam at the frequency of the TTL pulse. Because only the zero-order beam406 is caused to fall on the sample chamber 412, the AOM 400 acts as anefficient chopper for the input laser beam 404.

Another approach for achieving chopping is through a MEMS cantileverapparatus 500 that is mechanically scanned across a laser beam 502 byelectrostatic actuation, as shown in FIG. 5. A voltage at the modulationfrequency is applied to a cantilever beam 504 with respect to a bottomground plate 506, wherein the changing electrostatic force between thecantilever 504 and the plate 506 causes the cantilever 504 to vibrateacross the path of the laser beam 502, which is incident on the sampleholder. As can be seen, the cantilever is initially positioned so as toblock the path of the laser beam 502 when no voltage is applied thereto.Then, when a voltage is applied across the cantilever 504 and groundplate 506, the resulting attraction therebetween causes cantilever 504to deflect toward the ground plate 506, moving the cantilever 504 out ofthe path of beam 502. When the voltage is removed, the cantilever 504then return to its initial position to block the beam 502. It will alsobe appreciated that the cantilever apparatus 500 could also beconfigured to operate in reverse; i.e., applying a voltage causes thecantilever to deflect into the path of the beam 502 instead of out ofthe path of the beam.

Regardless of how the laser beam is chopped (using, for example, eitherof the two aforementioned techniques), it interacts with the sample andthe Raman signal is frequency filtered in the tunable Fabry Perotfilter, and the resulting intensity variation is detected by aphotodetector. The output of the photodetector (e.g., detector 212 inFIG. 2) is then demodulated at the reference frequency (of chopping)using a lock-in detection circuit (not shown), and the Raman spectrumfor the sample concerned is thus obtained. In still a furtherembodiment, the heterodyne technique (using an AOM or MEMS cantilever asa chopping mechanism) may be used in combination with the abovedescribed photon counting method.

As will thus be appreciated, the above described photo counting and/orheterodyne detection techniques address several problems associated withintegrated, hand-held Raman micro spectrometer devices, including forexample, power management on surface of sample chamber. Since Ramanprocesses are very weak, the samples need to be irradiated withconsiderably high laser power, resulting in an intensity of at leastabout 650 kW/cm² (for a 50 mW beam focused to a 100 μm diameter spot) atthe wall of the sample chamber. This may cause serious damage to thewalls of the sample chamber, as well as to liquid or solid samples dueto burning/melting/boiling. The proposed embodiments would mitigate thisrisk, since by chopping the laser beam using an acousto-optic modulatoror a mechanical chopper, the sample chamber is protected from continuousexposure to the laser radiation, thereby reducing heating up of thechamber and sample significantly.

Moreover, the embodiments of the invention also represent a low costsolution for power management, as they obviate the need for an expensivepulsed laser source, which is typically the most common solution toprevent unwarranted heating of the sample chamber walls and/or sampleitself. The scanning cantilever switch of FIG. 5 is also a low cost MEMSsolution for effectively modulating the source laser beam. As alsostated above, since Raman signals are extremely weak (e.g., about 100 pWat most), SNR is always an issue, especially for trace sample detection.Heterodyne detection can thus be used to improve SNR by more than afactor of 100. Further, since a Raman micro spectrometer is be aportable instrument to be operated in the field, stray ambient light isa big problem, particularly considering the fact that Raman signals arevery weak. Heterodyne detection is therefore very useful for extractingextremely low signal levels from large background noise, since thedetection is carried out specifically at the reference modulationfrequency.

While the invention has been described with reference to exemplaryembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. A method of Raman detection for a portable, integrated spectrometerinstrument, the method comprising: directing Raman scattered photons bya sample to an avalanche photodiode (APD), said APD configured togenerate an output signal responsive to the intensity of said Ramanscattered photons incident thereon; amplifying said output signal ofsaid APD and passing said amplified output signal through adiscriminator so as to reject at least one or more of amplifier noiseand dark noise; and counting a number of discrete output pulses within aset operational range of said discriminator so as to determine a numberof photons detected by said APD.
 2. The method of claim 1, furthercomprising cooling said APD by thermoelectric cooling.
 3. The method ofclaim 2, wherein said APD is biased in a sub-Geiger mode of operation.4. The method of claim 1, further comprising passing said output signalof said APD through a preamplifier so as to shape an output currentpulse from said APD to a NIM standard pulse.
 5. The method of claim 1,further comprising chopping an input optical beam prior to Ramanscattering.
 6. The method of claim 5, wherein said chopping isimplemented through frequency modulation of an acousto-optic modulator.7. The method of claim 5, wherein said chopping is implemented throughfrequency modulation of a deflectable cantilever positioned within thepath of said input optical beam.
 8. An apparatus for Raman detectionwithin a portable, integrated spectrometer instrument, comprising: anavalanche photodiode (APD) integrated on a chip, said APD configured toreceive Raman scattered photons by a sample and generate an outputsignal responsive to the intensity of said Raman scattered photonsincident thereon; an amplifier integrated on said chip, said amplifierconfigured to amplify said output signal of said APD and pass saidamplified output signal through a discriminator integrated on said chip,so as to reject at least one or more of amplifier noise and dark noise;and a digitizer integrated on said chip, configured to count a number ofdiscrete output pulses within a set operational range of saiddiscriminator so as to determine a number of photons detected by saidAPD.
 9. The apparatus of claim 8, wherein said APD is cooled bythermoelectric cooling.
 10. The apparatus of claim 8, wherein said APDis biased in a sub-Geiger mode of operation.
 11. The apparatus of claim8, further comprising a preamplifier integrated on said chip, saidpreamplifier configured to shape an output current pulse from said APDto a NIM standard pulse.
 12. The apparatus of claim 8, furthercomprising a beam-interrupting mechanism configured to chop an inputoptical beam prior to Raman scattering.
 13. The apparatus of claim 12,wherein said beam-interrupting mechanism comprises a frequency modulatedacousto-optic modulator.
 14. The apparatus of claim 12, wherein saidbeam-interrupting mechanism comprises a frequency modulated, deflectablecantilever positioned within the path of said input optical beam.15.-26. (canceled)